The so-far known processes for producing (S)-azetidine-2-carboxylic acid are as follows: (1) The process which comprises reacting L-2,4-diaminobutyric acid with hydrochloric acid and nitrous acid to give L-2-chloro-4-aminobutyric acid and then subjecting the same to heat treatment in barium hydroxide to give L-azetidine-2-carboxylic acid (The Biochemical Journal, vol. 64, pp. 323 to 332, 1956); (2) The process which comprises reacting-.gamma.-butyrolactone with bromine in the presence of red phosphorus and treating the reaction product with benzyl alcohol saturated with hydrogen chloride gas to give benzyl DL-2,4-dibromobutyrate, reacting the same with benzhydrylamine to give benzyl DL-N-diphenylmethylazetidine-2-carboxylate, reducing this with hydrogen in methanol in the presence of palladium carbon to give DL-azetidine-2-carboxylic acid, and then subjecting it to reacting with benzyloxycarbonyl chloride (ZCl) to give DL-N-Z-azetidine-2-carboxylic acid, subjecting the latter to optical resolution using L-thyrosine hydrazide to give L-form, namely (S)-N-Z-azetidine-2-carboxylic acid (wherein Z represents a benzyloxycarbonyl group; hereinafter the same shall apply), and lastly reducing the same again with hydrogen in methanol in the presence of palladium carbon to give L-azetidine-2-carboxylic acid (Journal of Heterocyclic Chemistry, vol. 6, pp. 435 to 437, pp. 993 to 994, 1969); and (3) The process which comprises S-alkylating L-N-tosyl-methionine to give L-N-tosyl-methioninesulfonium salt, heating it in an aqueous solution of sodium hydroxide for conversion to L-N-tosyl-.alpha.-amino-.gamma.-butyrolactone, treating the latter with a gaseous hydrogen halide in an alcohol to give alkyl L-N-tosyl-2-amino-4-halobutyrate, treating this with sodium hydride in dimethylformamide to give (S)-N-tosyl-azetidine-2-carboxylic acid, and thereby effecting the cyclization reaction, and detosylating it with metallic sodium in liquid ammonia to give (S)-azetidine-2-carboxylic acid (Japanese Patent Application sho-49-14457, Chemistry Letters, pp. 5 to 6, 1973).